configure.ac: don't require GNU-specific files when running automake

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -durN madplay-0.15.2b-orig/configure.ac madplay-0.15.2b/configure.ac
--- madplay-0.15.2b-orig/configure.ac	2019-02-14 21:34:01.507212449 +0100
+++ madplay-0.15.2b/configure.ac	2019-02-14 21:34:23.439336353 +0100
@@ -26,7 +26,7 @@
 
 AC_CONFIG_SRCDIR([madplay.c])
 
-AM_INIT_AUTOMAKE
+AM_INIT_AUTOMAKE([foreign])
 
 AM_CONFIG_HEADER([config.h])
 
